Cotton crops suffer from environmental stress, particularly extreme temperatures, which cause significant reduction in yields. Plants are more sensitive to high temperatures stress during the reproductive stage, when pollination, pollen tube growth, and ovule fertilization occur. High temperature results in decreased seed and boll size and lower boll retention. The average daily maximum temperatures during flowering and boll development normally exceed 35^oC well above the optimum for cotton growth, and there is a correlation between high temperatures during reproductive development and low yields. One of the principal problems about research in environmental stress physiology is that ambient temperatures are used to evaluate effects on yields, while actual temperatures in the canopy where bolls develop are different. Little is known about the effects that temperatures at different depths in the canopy may have on leaf and boll development, and therefore on yield. The influence of canopy microclimate on boll growth and, consequently on seed and fiber yield has not yet been described. It is hypothesized that leaf, boll, and air temperatures within the canopy are better correlated with boll growth and yield than ambient air temperatures. A field was conducted in Fayetteville, Arkansas with two planting dates to give different temperature at the same sampling morphological stage, and two nitrogen regimes to give two different canopy structures. Measurements were made at weekly intervals at two positions in the canopy of boll surface and internal temperatures, the microclimate surrounding the boll, and compared with ambient and canopy temperature. The effect of temperature on the boll was recorded by measurements of boll carbohydrates, boll respiration, and dry weight. Data are being analyzed and results will be presented.